I'm 33 and I've gained more friends the older I've got. As a rough estimate I probably would say I have at least 20 or 30 people I could class as close friends.
In primary school I only had one or two friends. I was considered a weird, quiet kid by most of my peers. Gained a few more friends in my teens. In adult life I've made loads more friends mainly from places I've worked or mutual friends of existing friends. I'm still weird, perhaps less quiet than I was though.
I do have some friends in a similar situation to your mate. I've been told by at least 3 of mine that I'm their only friend. Others I've introduced to some of my different groups of friends and they've got on well but it doesn't always work. I often find in social situations I'm the link between people and they won't talk or meet up without me present.